docs: def get_range(self):
          """
          Return the possible types for this parameter's value.
  
          (I.e. return {name: <class>} for all classes that are
          concrete_descendents() of self.class_.)
  
          Only classes from modules that have been imported are added
          (see concrete_descendents()).
          """
          classes = concrete_descendents(self.class_)
          d=OrderedDict((name,class_) for name,class_ in classes.items())
          if self.allow_None:
              d['None']=None
          return d